Solve for $a$ : $18 = a + 9$
Solution: Subtract $9$ from both sides: $ \begin{eqnarray} \\ \hphantom{18 {- 9}} &\hphantom{=}& \hphantom{a} \\ 18 &=& a + 9 \\ \\ {-9} && {-9} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 18 {- 9} &=& a \\ \end{eqnarray} $ Simplify. $ a = 9$